The present invention relates to communication systems. More particularly, and not by way of limitation, the present invention is directed to a system and method for improving network performance in an Internet Protocol (IP) based network by providing path symmetry, resilience, and scalability.
Telecommunication site operators are increasingly deploying packet-switched networks to carry voice traffic over IP instead of traditional circuit-switched networks. In this process, analog voice calls are digitized or otherwise transcoded using Media Gateways (MGWs) and encapsulated in IP packets. These IP packets are transported over a network consisting of the operator's own IP network (hereinafter called the “site network”) and over its provider's networks to other MGWs before being delivered to the called party. The traffic handled by the MGWs can be voice traffic or other media. In the description herein, this type of traffic is referred to as Circuit Switched payload (CS Payload) traffic to identify the traffic handling requirements of such traffic.
An IP network managed by a single administrative domain is called an Autonomous System (AS). Connectivity between autonomous systems is provided by a routing protocol called Border Gateway Protocol (BGP), whereas connectivity within an autonomous system is managed by Interior Gateway Protocols (IGPs) such as Open Shortest Path First (OSPF) and Intermediate System to Intermediate System (IS-IS) routing protocols. BGP enables the exchange of routing information based on administrative policies. IGPs, on the other hand, compute the best paths internal to the autonomous system based primarily on a given metric. When more than one path exists to a given destination with the same cost, all of the paths can be used to forward traffic through the autonomous system. This is called Equal Cost Multi-Path (ECMP). ECMP can be used, for example, to provision multiple links between two routers to address bandwidth limitations of an existing link.
BGP can be deployed in two modes: External BGP (eBGP), which is used to interconnect two autonomous systems, and Internal BGP (iBGP), which is used between two border routers of the same autonomous system to exchange BGP information learned by eBGP-speaking routers. Additionally, to support large autonomous systems, an autonomous system can be divided into multiple internal autonomous systems, called Confederations (BGP_CFD), and eBGP can be used to exchange routing between confederations. This process is referred to as “eiBGP” herein.
FIG. 1 is a simplified block diagram of an existing network topology connecting an IP Site Network 11 operated by a telecommunication site operator to an IP transport network 12. The Site Network includes a Host/MGW 13, a LAN 14, and a Site Edge Router (SER) 15. The Site Network is connected to the transport network using two routers. The SER 15 is the router on the Site Network's border and belongs to the telecommunication site. A Provider Edge (PE) router 16 is on the edge of the transport network. Thus, the PE router connects to the Site Network via the SER.
Site Networks require the network nodes to be resilient and scalable. IP router resiliency is typically achieved using the Virtual Router Redundancy Protocol (VRRP). VRRP allows two IP routers to backup each other when connected over a Local Area Network (LAN). The hosts and MGWs of the site have a default route to the SER. The SER runs a routing protocol to the PE router. The SER also has a connected route for its network to the host. If there are application nodes internal to the site that also run routing protocols, the SER may run an IGP on the host LAN. In general, an SER can be connected to multiple host LANs, even though only one such LAN is shown in FIG. 1.
IP packets are transported in packet-switched networks primarily based on packet destination addresses. Packets exchanged between two media gateways, for example, are routed independently purely based on the destination media gateway address. As a result, there are no guarantees that all traffic belonging to a single voice call will take the same route in each direction. This asymmetric packet forwarding poses several problems in adapting IP networks to carry CS Payload traffic. First, the propagation delay is different in each direction. Hence the latency perceived by the two parties of a voice call is different, resulting in unsatisfactory conversational experience. Second, when a link or router fails, asymmetric routing affects double the number of voice calls compared to circuit-switched forwarding where paths can be set to be symmetric.
FIG. 2 is a simplified block diagram of an existing asymmetric network topology connecting an MGW 21 to a transport network 12 through routers on two LAN interfaces, LAN1 and LAN2. The routers include SER1 22 and PE1 23 on the LAN1 interface, and SER2 24 and PE2 25 on the LAN2 interface. In the illustrated example, the MGW is servicing ten (10) calls with asymmetric outbound and inbound paths as shown. If either of the two links fails, or if any of the routers fail, all of the calls are dropped. If the paths taken by the bidirectional calls are symmetric, only those calls using the failed link or router would be dropped, leaving the remaining calls unaffected.
The problem is exacerbated when ECMP is used to achieve scalability of bandwidth with multiple low bandwidth links and routers. With ECMP, packets belonging to the same call flow are forwarded to the same nexthop router, but there is again no guarantee that the outbound and inbound flow of a given call will follow the same path.
The only known solutions to achieve path symmetry is by building an IP network using link layer technologies that offer path symmetry, such as Asynchronous Transfer Mode (ATM), Frame Relay, or more recently Multi-Protocol Label Switching (MPLS). These solutions require that the entire network between MGWs be built using similar circuit-switched technology. Increases in traffic load are addressed by procuring high bandwidth links and more powerful routers and switches. This results in a disproportionate increase in capital expenditure costs for the site operator compared to increases in network capacity.